CPC Best Plant Conservation Practices

to Support Species Survival in the Wild

Key Messages about Genetics Related to Using a Collection
  • Give your reintroduced population a good chance for survival by starting with adequate numbers of plants or seeds at the beginning.
  • Determine the source of the plants or seeds.
  • Determine whether the source should consist of seeds or plants from a single population or from mixed populations.

The larger the founding population the greater the chance of success in establishing self-sustaining populations.

  • In general, the larger the founding population, the greater the chance of it surviving to become an established, self-sustaining population will be (Guerrant 1996). Use as many individuals as is feasible (50 individuals or more) for a reintroduction or conservation translocation (Guerrant 1996, Albrecht and Maschinski 2012, Albrecht et al. 2019).
  • To compensate for propagule losses due to mortality during reintroduction, start with an estimate of desired numbers of individuals surviving to reproduction in a new founding population. Then, account for expected losses during establishment. Some of these calculated losses can be mitigated by maintaining backup clonal material. The greater number of individuals (from diverse maternal lines), the greater chance that diverse genes are represented in the reintroduced population.
  • When growing the material for purposes of a reintroduction or other conservation translocations, keep in mind the reproductive biology of the species. For example, obtaining 10 female plants of a dioecious species may require planting twice as many seeds as the expected number of seeds that germinate if the sex ratio is 50:50.

Ascertain whether genetic studies are needed before conducting the reintroduction.

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Questions to Ask about When Genetic Studies are Needed

Assessing the genetic diversity of wild populations can reveal insights about the biology of the species, however genetic studies can be expensive and may not always be necessary. They can include either molecular work (genotyping, sequencing, genome or ploidy analysis) or common garden studies. These types of studies are advisable before collecting a rare species or before conducting a reintroduction if the wild populations have any of the following characteristics:

Within-population issues

  • Population has fewer than 50 individuals flowering and setting fruit.
  • The species is clonal.
  • Little or no viable seed is being set.
  • There are potential taxonomic concerns (taxonomic ambiguity, potential hybrids, or variation in ploidy).

Issues across the species’ range

  • The species is declining and little is known about the biology or life history of the species.
  • The species has highly fragmented and isolated populations.
  • The species looks different in different locations.
  • One or more populations of the species has distinct ecology from the majority of populations.

Using Genetics to Inform Restoration

Kim McCue, Shannon Felberg and Steve Blackwell Desert Botanic Garden

Review and plan the source material that will be appropriate to introduce to a particular site (Basey et al. 2015).

  • Identify the potential source material(s) available for conservation translocation. Note collection site ecological conditions, community structure, and proximity to the proposed recipient site (See CPC Best Practice Section, “Preparing the Reintroduction;” Maschinski et al. 2012).
  • Collect or retrieve from a seed bank the source material whose location has similar climatic and environmental conditions to the recipient site(s). Detailed information recorded on accession forms at time of the collection is essential for this evaluation.
  • The extent of gene flow between populations varies by species. Some may have isolated, locally adapted patches within a small area, whereas others may have great gene flow over great distances, therefore there isn’t a simple relationship between distance and genetic relatedness (Richards et al. 2016).
  • Use genetically heterogeneous founders to improve the ability to cope with varying conditions (Falk et al. 1996; Guerrant et al. 2004; Neale 2012). Theoretically, high levels of genetic diversity will equip the new population with adaptive potential needed to withstand stochastic and deterministic events including climate change and can defend against potential genetic pitfalls of small populations such as founder effect and inbreeding depression.

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What a Genetic Assessment Can Tell You and How That Information Can Be Applied

  • Molecular studies can help with determining:
    • Information about the biology of a species, such as mating system and neighborhood size
    • Levels of overall neutral diversity represented within the population. Although this does not always equate to amount of resilience a population has in response to stressors such as drought or disease, it can identify issues such as bottlenecks, genetic drift, or a limited number of founders.
    • A quantified level of differentiation among populations, which is important for prioritizing populations for collection and identifying appropriate sources for reintroductions
    • Effective population size Ne. This helps identify how many unrelated individuals are reproducing in the population.
    • Degree of clonal/asexual growth to ensure you collect the maximum number of unrelated individuals possible
    • Historic migration rates to identify natural levels of gene flow
    • Historic levels of inbreeding to minimize potential inbreeding depression in collections and reintroductions
  • Genomic studies can aid in determination of:
    • Traditional measures listed for molecular studies above
    • Location of genes along chromosomes
    • Linking genetic fingerprint/genetic loci to ecological attributes
    • Loci that may be subject to differential selection across populations, which can be used to identify locally adapted populations and prioritize populations for collection
  • Common gardens can help:
    • Identify morphological or ecological differences between groups to ensure matching source to conservation translocation site
    • Quantify level of phenotypic differentiation among populations to identify potentially important adaptive differences
  • Breeding studies can help:
    • Identify self-incompatibility alleles that might limit mating
  • Flow cytometry can help:
    • Identify differences in ploidy/cytotypes that might lead to reproductive issues if mixed ploidy individuals are cross-pollinated

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Decide whether to use single source versus mixed populations.

  • Sometimes it may be appropriate to use a single-source population, while other times it may be appropriate to mix populations.
  • The decision of whether to mix source populations or keep separate should consider several factors: condition and context of the wild population(s), mating system, dispersal mode, ploidy level, and genetic structure. (See Figure 3.2, “Summary of Collecting Recommendations for Numbers of Populations to Sample,” and Figure 3.3, “Summary of Collecting Recommendations for Numbers of Individuals to Sample within a Population.”)
  • Traditionally, it is recommended to use founders from only a single wild population that is ecologically similar to the recipient site in order to preserve locally adapted genes. For example, if the species is an obligate outcrosser and is locally adapted to a site at very fine scale, then mixing populations may cause outbreeding depression (Neale 2012). This is especially true if there are known genetic differences between existing populations or if populations have more than 100 individuals, have distinct ecology, and have been separated for more than 20 generations (Frankham et al. 2011).
  • Mixing source material in a restoration may be necessary if there is no appropriate ecological recipient site that matches the site where the population currently grows, if the available source material is limited, or if there is evidence of low genetic diversity or inbreeding depression in the source population (Dalrymple et al. 2012; Neale 2012). We recommend mixing source material if the taxon has extant populations of less than 100 individuals with no chromosomal differences, no distinct ecological differences, and if populations have been separated less than 500 years (Frankham et al. 2011).
  • If mixing sources, keep track of each individual source through collection, production, and conservation translocation to allow for rapid response should any issues arise.

Use founders with evenly represented family lines.

  • Collect and maintain seeds from each maternal line separately. In this way, it is possible to know and intentionally control even representation of the different founders.
  • Minimize ‘‘unconscious’’ selection during seed increases or augmentation of natural populations. Note that variation in germination and growth of maternal lines should be expected. Resist the temptation to over-represent the winners—those abundantly available, vigorously growing maternal lines that may skew the diversity of the population—but rather consciously maintain even family line representation (Guerrant et al. 2004; McKay et al. 2005).

Consider genetic rescue when appropriate.

  • When a wild or reintroduced population has low genetic diversity and signs of inbreeding depression, consider genetic rescue (Frankham 2015).
  • Infusing new genetic stock into a wild or reintroduced population (genetic rescue) may be necessary to overcome detrimental effects of inbreeding (Frankham 2015). Introducing new individuals or genes (from pollen) could increase genetic diversity and fitness of a small, inbred population (DeMauro 1993; White et al. 2018).
  • Aim to release equal numbers of individuals from each source population early in the reintroduction to promote balanced admixture in the descendant population (Havens et al. 2004; White et al. 2018).
  • For species critically imperiled by threats that are genetically linked, genetic rescue may also comprise insertion of advantageous genes as is being done in crop development (Rinaldo and Ayliffe 2015).

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